The average adult body contains about 8 to 12 pints of blood. Blood is distributed through more than 70,000 miles of tubes known as the vascular system. If these tubes were lined up end to end, they would reach between New York City and San Francisco over 24 times. Blood is the transporting fluid for the body. Blood's composition is complex and essential to sustaining life.

The phlebotomist must have an understanding of the circulation of blood and its composition and functions. In addition to the cellular and liquid composition of blood, the phlebotomist must know how the closed circuit of blood vessels transports blood. Knowing the location of blood vessels, especially the most commonly used arm veins, and the composition of blood is essential to performing venipuncture.